The present disclosure provides methods and kits for treating and classifying individuals at risk of or suffering from a neurological and/or mitochondrial dysfunction or disorder. In general, the individuals are treated and/or classified based on the presence of a loss-of-function mutation in nuclear DNA encoding one or more proteins selected from the group consisting of ALDH1L1, ALDH1L2, FOLR1, FPGS, GCSH, GLDC, MTHFD1, MTHFD1L, MTHFD2, MTHFD2L, MTHFS, MTRR, SHMT1, SHMT2 and SLC25A32. Treatment involves the administration of a therapeutically effective amount of folinic acid, glycine or a pharmaceutically acceptable salt thereof.
